1. Field of the Invention
The present invention relates to a semiconductor pressure sensor device, and more particularly, it relates to a semiconductor pressure sensor device employed for an application of a micro pressure such as tank internal pressure sensors detecting gasoline leakages of vehicles, and so on.
2. Description of the Background Art
The semiconductor pressure sensor device detects a pressure employing an effect converting a pressure added to a semiconductor into an electronic signal. A device which includes a semiconductor sensor substrate having a diaphragm of appropriate thickness for generating a predetermined electronic signal corresponding to a predetermined pressure and a glass base as a support member to fix the semiconductor sensor substrate is known as such a semiconductor pressure sensor device.
There is a sensor such as shown in FIG. 1 of Japanese Patent Application Laid-Open No. 8-21774 (1996) (pp. 3 and 4) as the semiconductor pressure sensor having a composition described above. The semiconductor pressure sensor which is known conventionally employs a circular diaphragm changing corresponding to a pressure difference on its both surfaces. This diaphragm is formed in a square semiconductor sensor substrate composed of a single-crystal silicon substrate having a stress sensor formed on one surface of it. Four strain gage elements are formed on the periphery of the diaphragm and detect a stress on the diaphragm. These strain gage elements indicates a piezoresistance characteristic and the resistance changes corresponding to the stress experienced by the sensor. Furthermore, a circular cavity is formed by a silicon etching in the surface opposite to the surface where the strain gage elements are placed to form the diaphragm. Moreover, a support member composed of a borosilicate glass is joined on the other main surface of the semiconductor sensor substrate.
In the meantime, the support member has a through-hole which becomes a circular stress leading-in hole in the proximity of its center part and is connected with the surface opposite to the surface where the strain gage elements are formed. Moreover, a circular supporting part surrounding the diaphragm and moreover being concentric with this is placed in higher level than its periphery by several μm on a bonding region of the support member facing with a thick part surrounding the diaphragm of the semiconductor sensor substrate. According to this, a shape of the bonding region becomes circular and a zero shift phenomenon caused by a condition that the shape of the bonding region with the support member is not symmetrical to a shape of the diaphragm is prevented.
In case of a silicon etching described above, an etching velocity changes corresponding to a temperature of an etching solution and a concentration of an etchant, thus a homogenization of the solution temperature and the etchant concentration is provided by rotating the silicon substrate in the etching solution, however, there is a tendency that a turbulent flow occurs in a cavity adjacent to the diaphragm in case that an etching amount of the silicon in a depth direction increases, a temperature difference and an etchant concentration difference come to occur easily, and then a thickness of a periphery part is reduced in size as compared with a central part of the diaphragm. The thickness of the silicon substrate is 400 μm, and the thickness of the diaphragm is approximately 18 μm, thus a reasonable technique is necessary to control the thickness of the diaphragm in the conventional technique described above also, however, it is necessary to reduce moreover the thickness of the diaphragm to obtain a stable signal with a fair S/N Ratio in a micro pressure measurement, and consequently, the etching amount of the silicon gets to increase, and according to this, the thickness of the diaphragm varies furthermore widely, and at worst, there is a case that a hole is formed in the diaphragm in part.
In the meantime, when the thickness of the silicon substrate is reduced in advance, the thickness of the diaphragm can be reduced by reason that the etching amount decreases and the difference lessens, however in the meantime, the thickness of the thick part is also reduced. When the thick part is thick enough, a bonding stress with the support member can be absorbed and thus an influence upon the diaphragm can be curbed, however, when the thickness of the thick part is reduced, a problem arises that the bonding stress cannot be absorbed in the thick part sufficiently, the diaphragm provides a larger strain, a resistance value of the strain gage element formed on the semiconductor sensor substrate changes, and these conditions cause a drop of an initial characteristic. Consequently, 400 μm in thickness is necessary for the silicon substrate.
In case of Japanese Patent Application Laid-Open No. 8-21774, the zero shift phenomenon caused by a condition that the shape of the bonding region with the support member is not symmetrical to the shape of the diaphragm is prevented by forming the circular supporting part surrounding the diaphragm and moreover being concentric with this higher than its periphery on the bonding region of the support member facing with the thick part surrounding the diaphragm of the semiconductor sensor substrate and also making the shape of the bonding region circular, however, the subject described above is not recognized. Furthermore, in case that a slippage of a position unavoidable upon a manufacturing process occurs when the semiconductor sensor substrate and the support member are joined with each other, the symmetry of the shape of the bonding region with the support member to the shape of the diaphragm cannot still be secured, and thus the occurrence of the zero shift phenomenon is concerned.